This invention relates generally to zirconcontaining foundry sand and, more particularly, to a process for treating zircon-containing foundry sand with an alkali metal silicate to improve the tensile strength of resin-shell molds or cores in which the treated sand is used.
Zircon-containing foundry sands include mineral sands containing as little as 15% zircon to nearly 100% zircon with only trace amounts of other components. The zircon component of the sand is believed to provide thermal stability in foundry applications. Consequently, as the amount of zircon present in the foundry sand increases so does the thermal stability. Zircon-containing foundry sand is widely used as the major component of high-performance resin-shell molds and cores because it can be bonded and worked in the same manner as silica sands, has high thermal stability, and provides improved casting surface finishes relative to silica sands. Zircon-containing resin-shell molds and cores are conventionally prepared by contacting a mixture of resin and sand or, preferably, a resin-coated sand with a preheated metal pattern. The resin, upon curing, acts to bind the particles of sand in the form of the metal pattern.
While zircon-containing resin-shell molds and cores exhibit superior properties relative to silica resin molds, numerous defects can still occur during the casting process. Since the resin mold must be strong enough to contain the molten metal until it solidifies, sufficient resin binder must be present so that the resin-shell mold will maintain structural integrity during the solidification process. Traditionally about 3% by weight of resin is the minimum required for a sufficiently strong bond. However, at amounts of resin high enough to insure structural integrity of the mold, defects caused by the decomposition of the resin can occur. These "gas defects" are caused by the penetration of gaseous decomposition products into the molten or solidifying metal and result in pin holes and scarring of the resulting metal shape. Furthermore, since the mold must collapse after solidification, high amounts of resin can at least partially prevent collapse of the mold and cause skakeout problems.
This invention provides for a process for treating zircon-containing foundry sand which provides resin molds containing the treated sand with exceptionally high strength at low resin loading.